This invention relates in general to methods for hydroforming hollow or closed channel workpieces to achieve desired shapes. In particular, this invention relates to an improved method for performing a hydroforming operation that resists the tendency of the workpiece to be deformed inwardly upon itself when the closed channel workpiece is initially being enclosed by a pair of opposed hydroforming dies of a hydroforming apparatus.
Hydroforming is a well known metal working operation that uses pressurized fluid to deform a closed channel workpiece, such as a tubular member, outwardly into conformance with a die cavity having a desired shape. A typical hydroforming apparatus includes a frame having two or more hydroforming die sections that are supported thereon for relative movement between opened and closed positions. The die sections have cooperating recesses formed therein that together define a die cavity having a shape that corresponds to a desired final shape for the workpiece. When moved to the opened position, the die sections are spaced apart from one another to allow a workpiece to be inserted within or removed from the die cavity. When moved to the closed position, the die sections are disposed adjacent to one another so as to enclose the workpiece within the die cavity. Thereafter, the workpiece is filled with a fluid, typically a relatively incompressible liquid, such as water. The pressure of the fluid within the workpiece is increased to such a magnitude that the workpiece is expanded outwardly into conformance with the die cavity. As a result, the workpiece is deformed or expanded into the desired final shape.
Although the die cavity is usually somewhat larger than the workpiece to be hydroformed, the movement of the two die sections from the opened position to the closed position may, in some instances, cause some mechanical deformation of the hollow workpiece. A relatively small amount of this mechanical deformation is usually acceptable. However, in some instances, the amount of this mechanical deformation of the hollow workpiece is relatively large. Such relatively large deformation of the hollow workpiece can be undesirable because it may result in undesirable work hardening of the workpiece and inhibit the free flow of the material of the workpiece during the subsequent performance of the hydroforming operation. Thus, it would be desirable to provide an improved method for performing a hydroforming operation that resists the tendency of the workpiece to be deformed inwardly upon itself when the closed channel workpiece is initially being enclosed by a pair of opposed hydroforming dies of a hydroforming apparatus.